High Quality Content by WIKIPEDIA articles! Long Valley Caldera is a depression in eastern California that is adjacent to Mammoth Mountain. The valley is one of the largest calderas on earth, measuring about 20 miles (32 km) long (east-west) and 11 miles (18 km) wide (north-south). The elevation of the floor of the caldera is 6,500 feet (2,000 m) in the east and 8,500 feet (2,600 m) in the west. The elevation of the caldera walls reach 9,800–11,500 feet (3,000–3,500 m), except in the east where the wall rises only 500 feet (150 m) to an elevation of 7,550 feet (2,300 m).